A mobile communication system having a mobile terminal and a base station for transmitting wireless signals with the mobile terminal has significantly developed centering on a portable telephone system. Above all, a system by a CDMA (Code Division Multiple Access) system is expected to spread worldwide in the future. According to the CDMA system portable telephone system, a portable telephone terminal (mobile terminal) is requested to change transmission power according to communication environment, such as a distance to the base station, behind buildings, or the like. In this case, in either a high output time or a low output time, it requires an amplifying apparatus for a portable telephone terminal constituting a power source of a battery with a high efficiency, i.e., a high ratio of consumption power to transmission power.
Generally, according to a high frequency signal amplification apparatus using a semiconductor device, the larger the output level, the more the efficiency is to be improved. The efficiency becomes the highest at a vicinity requesting the saturated output, i.e., the maximum possible output. The level of the saturated output depends on a size of a semiconductor device. Therefore, an amplifying apparatus with a low saturation level is produced by constituting a small semiconductor device in order to improve the efficiency in a low output time, an output necessary in a high output time is not supported. Further, an amplifying apparatus supports a high efficiency in a high output time with a large semiconductor device, the efficiency is significantly lowered at a low output time.
It is difficult to realize the high efficiency at both of a high output time and a low output time by a single amplifying apparatus. Therefore, an amplifying apparatus adopting a method for preparing semiconductor devices of different sizes for high output and for low output by switching therebetween in accordance with the output level is disclosed in, for example, JP-A-115331/1995.
FIG. 8 shows an example of a conventional amplifying apparatus switching in an output stage in accordance with an output level described in the above-described fabrication. A high frequency signal inputted from a high frequency signal input terminal 301, is inputted to a switching circuit 303 for switching paths via an initial stage amplifier 304 and a second stage amplifier 305 (constituting a driving stage). The switching circuit 303 on an input side and a switching circuit 306 on an output side select any one of a path of (1) only a simple transmission line 307, (2) a path passing through an output stage by one stage of an amplifier 308 and (3) a path passing through an output stage constituted by two stages of amplifiers 309 and 310 to output a high frequency signal to a high frequency signal output terminal 302. When the path of only the simple transmission line 307 is selected, the amplifier 305 becomes an output stage. The output stages of the respective paths are provided with maximum output powers different from each other. Further, a power source to the driving stage amplifiers 304 and 305 is given from a power source circuit 311, and a bias power source for operating only the output stage amplifier of the selected path, is given by a bias power source circuit 312.
According to the above-described conventional technology example, when the output stage is changed by switching the path, a length of a signal path is changed at the respective time. Further, a size of a semiconductor device used and a number of pieces thereof are changed such that the phase of an output signal is significantly changed. According to a simulation and experiments carried out by the inventors, it has been found that when the saturated power on the high output side is set to +30 dBm and the saturated power on the low output side is set to +20 dBm so as to provide an output difference of 10:1, a phase on the side of the high output is shifted by substantially 100 degrees.
According to a respective system of the GSM (Global System for Mobile communication), the PDC (Personal Digital Cellular), the N-CDMA (narrowband-CDMA), or the like adopted in a conventional portable telephone system, a base station uses a decoding system for decoding only a slot at the time point, no problem occurs even when the output stage path is switched and the phase is significantly changed before and after the time point.
However, a W-CDMA (Wideband-CDMA) system of the next generation, with regard to decoding, applies a technology of WMSA (Weighted Multi-Slot Averaging) which takes not only the slot at the current time point but also slots therebefore and thereafter into consideration. Therefore, an inconvenience occurs when the phase is significantly changed before and after switching the paths and accurate decoding cannot be carried out.
It is an object of the invention to provide a high frequency amplifying apparatus capable of reducing a change in a phase caused when the apparatus is switched to a different output level.